There are two general types of no-return loop or returnless fuel injection systems for a combustion engine. The first type, referred to as a “T” configuration, is used in fuel system applications where the fuel pressure within an injector fuel rail is held constant regardless of the mass fuel amount flowing through the injectors. The second type is referred to as a “parallel” configuration and is particularly popular in fuel systems requiring varying fuel pressure within the injector fuel rail dependent upon a particular engine transient. For instance, turbo-charged engines often require injector fuel rail pressures at wide open throttle conditions which are twice that at idle or engine coasting conditions. Both types commonly utilize a cycling or variable speed fuel pump which varies and controls fuel pressure via a pressure signal generated at the fuel rail.
The “T” configuration 10, as best shown in FIG. 1 as prior art, supplies fuel to an injector fuel rail 12 through a flow check valve 14 at the outlet of a variable speed fuel pump 16. The flow check valve 14 will close when fuel pressure at the valve outlet exceeds the fuel pressure at the valve inlet or pump outlet 18. The flow check valve 14 will typically close when the engine is shut-off, thereby, preventing fuel vaporization and preserving liquid fuel and pressure within the rail 12 for reliable engine start-up. Orientated between the flow check valve 14 and the fuel rail 12 of the “T” configuration 10 is a pressure relief check valve 19 for bleeding fuel directly back to the fuel tank in the event the fuel rail and injectors are subject to an overpressure condition. The pressure relief check valve 19 is designed to typically open when fuel pressure at the fuel rail 12 or inlet 17 of the pressure relief check valve 19 exceeds a predetermined value which is higher than the normal operating pressure at the fuel rail 12.
For instance, an overpressure condition may be caused after engine shutdown, wherein the flow check valve 14 is closed and the resultant trapped fuel within the fuel rail 12 rises in pressure with increasing fuel temperature, possibly heated by the residual heat emanating from the hot engine or surrounding environment. Yet another scenario of an overpressure condition may be caused by a slow response time of the variable speed pump. For instance, when an engine running at wide open throttle is immediately decelerated into a coasting condition, the injectors may thus close for seconds at a time. This could cause a pressure spike if the variable speed fuel pump can not immediately respond thus the pressure relief check valve will open to relieve fuel pressure at the rail.
Unfortunately, because the pressure relief check valve is referenced to tank pressure as opposed to pump output pressure, the relief set pressure of the “T” configuration utilizing a variable speed fuel pump must be set well above system operating pressure. As a result, the range of pressure control within the fuel rail is limited and the fuel injectors exposed to higher fuel pressure are more likely to leak when the engine is shut-down. A second disadvantage of the typical “T” configuration is that a separate bypass line and associated fittings are required thus increasing the manufacturing costs and assembly required. The previously described “T” configuration also has a disadvantage of returning fuel overage directly to the fuel tank which may result, particularly under high temperature conditions, in the fuel pump continuously pumping fuel through the pressure relief check valve and back into the fuel tank.
Another known aspect of the “T” configuration fuel system utilizes a constant speed fuel pump in place of the variable speed pump previously described. To control fuel pressure, typically at about 300 kPa operating pressure for a typical engine application, the constant speed system utilizes a pressure regulation valve in place of the pressure relief valve 19 of the variable speed system. Unfortunately, when the engine is shutdown and the fuel system remains pressurized at operating pressure, any increase in ambient temperatures (i.e. residual heat from a hot engine) will cause the trapped fuel to rise in pressure. This rise immediately causes the pressure regulation valve to open emitting a controlled amount of fuel from the system to lower fuel pressure. Should ambient temperatures decrease, the system pressure of the trapped fuel will fall substantially below system operating pressure. Subsequently, any further fuel temperature increases in the system or fuel rail at the lower pressures will produce vapor in the system. When starting the engine, the regulation valve will open preventing the fuel rail from exceeding, even for a limited period of time, the relatively low system operating pressure (i.e. 300 kPa). Consequently, without some degree of a pressure spike during engine starts, the vapor will not collapse back into liquid fuel and a prolonged or rough engine start will result.
The second or “parallel” configuration, as disclosed in U.S. Pat. No. 5,361,742 (Briggs et al.) and U.S. Pat. No. 5,477,829 (Hassinger et al.), which is probably the most current type of fuel injection system, also utilizes a variable speed fuel pump which varies speed and thus fuel flow based on a fuel pressure input signal from the fuel rail. Unlike the “T” configuration, the “parallel” configuration utilizes a flow check valve and a pressure relief check valve orientated in parallel to one another at the outlet of the pump. During operation of a combustion engine employing the “parallel” configuration of the no-return loop fuel injection system, the flow check valve at the outlet of the fuel pump opens with minimal differential pressure when fuel is supplied to the fuel injector rail, and closes to prevent reverse flow of fuel when the pressure at the flow check valve outlet (or pressure at the rail) is greater than the outlet pressure at the pump (or inlet pressure to the flow check valve). If the pressure at the outlet of the flow check valve exceeds a predetermined value referenced to the outlet of the pump usually during long deceleration periods, the parallel pressure relief check valve will open and fuel will reverse flow through the idle pump. To reduce this excessive fuel pressure at the rail, the normally closed pressure relief check valve opens from a normally closed position while the flow check valve remains closed. The pressure relief setpoint is greater than that of the flow check valve, protects the fuel rail from over-pressurization, and prevents fuel in the rail from vaporizing during engine shut down. When the pressure relief check valve is open, fuel bleeds back from the fuel rail and through the outlet side of the fuel pump. This “parallel” configuration contrasts with the pressure relief check valve of the “T” configuration where the opening setpoint pressure of the pressure relief check valve is above the maximum running pressure of the fuel rail and the fuel bleed back is not through the fuel pump.
Unfortunately, the parallel combination of the pressure relief check valve and the flow check valve requires many moving parts and thus is expensive to manufacture and maintain. Moreover, both valves are typically of a poppet design. The flow check valve has a ball bearing as a head which engages a seat under its own weight when closed. The pressure relief check valve is similar but typically is assisted by the force of a spring to further bias the ball bearing against the seat. Unfortunately, poppet valves are prone to wear and high frequency pressure fluctuations, as best shown in FIG. 7, which can degrade the smooth running performance of an engine.